Various miniature-type hearing aids are known, preferably of the type which can be carried on the ear of a wearer--see, for example, the referenced German Patent DE-PS No. 1 274 658, HEYNE. This publication describes a hearing aid which includes an extended audio duct so that the path of sound waves incident on the hearing aid from the exit opening therefrom to a sound processing station is prolonged. Extending the audio duct influences the frequency response. In very small or miniature hearing aids, the transmission characteristics of sound through the hearing aid duct should be matched to the frequency response and, unless the hearing aid duct has some predetermined minimum length, the low frequency response of the device as a whole may be unsatisfactory. It has therefore been proposed to use the housing of the hearing aid to extend the audio path by forming an audio duct therein in such a manner that the length of the audio path between the sound entrance opening and the exit opening from the audio duct is as long as possible. This hearing aid, while having improved frequency response, is so constructed that the hearing aid receiver and the sound extending path or duct form a single unit. It is not possible, therefore, to improve the frequency response of already existing hearing aids which do not have the extended sound path built into the unit itself, nor is it possible to match the frequency response of the overall sound transmission system formed by the hearing aid and the sound path to optimum values satisfactory to a user. Extending, for example, the sound duct, so that the audio path length is extended, is possible only by completely reworking the housing, which is difficult, and, since the housing is usually a plastic molding, practically impossible.